The present invention relates to a chainless transmission mechanism for bicycle, and more particularly to an improved structure of chainless transmission mechanism which is easy to install and durable in use, and which achieves a high performance.
Various chainless transmission mechanisms have been disclosed for use in bicycles for transmitting the driving power of the pedals to the rear-wheel hub. FIGS. 1 and 1A show a chainless transmission mechanism for a bicycle according to the prior art. This structure of chainless transmission mechanism comprises an axle housing 10 having a coupling flange 11 at one side fixedly fastened to a bottom bracket 2 of a bicycle frame 1, an end cap 14 covered on the axle housing 10 to hold a bottom bracket bearing axle 12 in the axle housing 10, a drive gear 13 fixedly mounted on the axle 12, a driven gear 18 mounted on an axle 4 of a rear-wheel hub 3, a shell 17 secured to the rear-wheel hub 3 by a nut 19, a first stay 5 connected between the axle housing 10 and the shell 17, a second stay 6 connected between the axle housing 10 and the rear-wheel hub 3, a transmission shaft 7 mounted in the first stay 5, a first transmission gear 15 and a second transmission gear 16 fixedly mounted on two opposite ends of the transmission shaft 7 and respectively meshed with the drive gear 13 and the driven gear 18. This structure of chainless transmission mechanism has drawbacks. Because the coupling flange 11 of the axle housing 10 which is fixedly secured to the bottom bracket 2 of the bicycle frame 1 is disposed at one side of the axle housing 10, the connecting area between the bottom bracket 2 and the axle housing 10 tends to be forced to deform or to break. Another drawback of this structure of chainless transmission mechanism is the complicated processing procedure of the coupling flange 11 of the axle housing 10. Still another drawback of this structure of chainless transmission mechanism is that the driven gear 18 tends to be forced out of place because it is fastened to the axle 4 of the rear-wheel hub 3 by a screw joint. Furthermore, because the gears 13;15;16;18 are bevel gear wheels, driving power may be intermittently transmitted from the bottom bracket bearing axle 12 to the axle 4 of the rear-wheel hub 3. This problem become worse when the teeth start to wear.
FIGS. 2 and 2A show another structure of chainless transmission mechanism for a bicycle according to the prior art. This structure of chainless transmission mechanism is substantially similar to that shown in FIG. 1. According to this structure, the bottom bracket 21 is shaped like a barrel; the axle housing 20 is coupled to the bottom bracket 21 and secured in place by a cap 22, which is fastened to the axle housing 20 by screws 23; the driven gear 18 is fixedly secured to the axle 4 of the rear-wheel hub 3 by a tightening-up screw 8. This structure of chainless transmission mechanism still has drawbacks. One drawback of this structure of chainless transmission mechanism is that the connecting area between the axle housing 20 and the cap 22 tends to be forced to deform, thereby causing the axle housing 20 to displace from the bottom bracket 21. Another drawback of this structure of chainless transmission mechanism is that driving power cannot be efficiently transmitted from the pedals to the axle 4 of the rearwheel hub 3 because the gears 13;15;16;18 are bevel gears. Furthermore, the driven gear 18 may be forced out of place easily because the tightening-up screw 8 wears quickly with use.